A number of exercise devices have been proposed in which the exerciser stands on the elongated device with his or her feet apart resting upon two spaced apart foot platforms separated by the center portion of the device which sits on a floor lying in a horizontal plane lower than the foot platforms on which the exerciser is standing. The lower surface of the center portion of the device is curved to enable the exerciser to rock from side to side or to tilt forth and back and thereby exercise his leg and back muscles and at the same time improve his balance. See, for example, U.S. Pat. Nos. 2,950,120; 3,188,087; 3,361,427 and 3,967,820. However, the commercial success of devices made according to the foregoing patents all of which have now expired is not known.